The invention relates to a complete knee joint prosthesis intended to be implanted with ablation of the posterior cruciate ligament.
Such a prosthesis, known as a postero-stabilized prosthesis, generally consists of an element which is anchored in the end of the femur and has a shield which reproduces the femoral condyles and the trochlea of the natural joint, an element which is anchored in the end of the tibia and has an essentially plane upper wall perpendicular to the longitudinal axis of the tibia, and an intermediate plate which provides for the sliding of these two elements in relation to one another.
The intermediate plate comprises, on the side of the femoral element, two lateral glenoid cavities which receive the femoral condyles and, on the side of the tibial element, an essentially plane surface which comes to rest on the upper wall of the tibial element.
The plate also comprises a cam-shaped stud intended to be engaged in the trochlea of the femoral element, and the femoral element comprises a transverse intercondylar bar intended to come into abutment against the stud. This coming into abutment prevents posterior subluxation of the tibia, replacing the posterior cruciate ligament which has been removed.
The femoral and tibial elements are in general made of metal, while the intermediate plate is made of a material which favours sliding, such as high-density polyethylene.
During the movements of the joint, the femoral and tibial elements repeatedly exert great forces on the intermediate plate, which are applied in an unbalanced manner to a greater or lesser degree. In the long term, these forces cause wear and flow of the polyethylene plate, resulting in imbalance of the joint and abnormal stresses of the ligaments, and which may lead to loosening of the bone elements. This problem arises all the more because wear and flow increase as the femoral and tibial elements develop play in relation to the plate.
Furthermore, the contact between the stud of the plate and the intercondylar bar of the femoral element occurs relatively violently, in the middle of flexion, and takes place over a limited surface area, which contributes to wear of the plate and risks loosening of the elements.
These prostheses also suffer from problems of femoropatellar alignment, of limited mobility during flexion and of the existence of stresses during rotation which expose the prosthetic elements to loosening.
Prostheses exist, in which the plate is mounted pivotably in relation to the tibial element.
This mobility of the plate makes it possible to limit wear and flow of plastic plate, as well as limit the problems of femoropatellar alignment, limited mobility during flexion, and the existence of stresses during rotation without, however, eliminating them.
The aim of the present invention is to remedy this fundamental disadvantage of knee prostheses by providing a prosthesis which leads to minimal wear of the intermediate plate in the long term while retaining a mobility of the femoral and tibial elements which correspond to that of the natural joint.